Immunological memory is a hallmark of antigen-specific T and B lymphocytes. In contrast, the innate immune system is known to launch rapid, non-specific effector responses, which are short-lived. However, recent studies have proposed a form of immunological memory in the innate immune system, where innate cells can ?remember? a pathogen encounter for several weeks to months. This phenomenon of ?trained immunity? has been documented for NK cells, but less is known about its role in monocytes and dendritic cells (DCs). Innate memory has been suggested to be mediated via epigenetic changes in myeloid cells, but there are several fundamental questions about the mechanisms of innate memory. In this proposal, we will address the following questions in the context of vaccination with the live attenuated yellow fever vaccine 17D (YFV-17D) or acute dengue viral infection in humans, and in mechanistic studies in mice: 1. Unlike memory T and B cells, most DC and monocyte subsets are believed to have a relatively short lifespan of a few days. So how can epigenetic changes acquired by such short-lived cells mediate innate memory? Can subsets of myeloid-derived cells persist for several weeks after infection or vaccination? 2. To what extent is so called ?innate memory? caused by the effects of an ongoing adaptive immune response (for example, via paracrine signaling), versus a cell intrinsic property of innate cells, similar to the classic phenomenon of immune memory exhibited by memory T or B cells? 3. Is there an enhanced response of DCs and monocytes, (similar to a memory response in the adaptive immune system), during secondary vaccination or infection? If so, what are the cellular and molecular mechanisms involved? We will address these questions in the following aims: Aim 1. Determining the innate response and regulatory landscape of DCs and monocytes in response to YFV-17D vaccination and dengue infection in humans. Aim 2: To determine the turnover rates of DCs and monocytes in response to YFV-17D vaccination in humans, using heavy water labeling. Aim 3: To define the mechanisms of innate memory induced by YFV-17D and adjuvants. Successful completion of these aims will further our mechanistic understanding of the phenomenon of innate memory, and offer novel strategies inducing broad and durable protection against diverse pathogens.